Generally speaking, paper products are formed by depositing a furnish comprising an aqueous slurry of papermaking fibers onto a forming section to form a paper web, and then dewatering the web to form a paper product. Various methods and machinery are used to form the paper web and to dewater the web. In papermaking processes to make tissue and towel products, for example, there are many ways to remove water in the processes, each with substantial variability. As a result, the paper products likewise have a large variability in properties.
One such method of dewatering a paper web is known in the art as conventional wet pressing (CWP). FIG. 1 shows an example of a CWP papermaking machine 100. Papermaking machine 100 has a forming section 110, which, in this case, is referred to in the art as a crescent former. The forming section 110 includes headbox 112 that deposits an aqueous furnish between a forming fabric 114 and a papermaking felt 116, thereby initially forming a nascent web 102. The forming fabric 114 is supported by rolls 122, 124, 126, 128. The papermaking felt 116 is supported by a forming roll 120. The nascent web 102 is transferred by the papermaking felt 116 along a felt run 118 that extends to a press roll 132 where the nascent web 102 is deposited onto a Yankee dryer section 140 in a press nip 130. The nascent web 102 is wet-pressed in the press nip 130 concurrently with the transfer to the Yankee dryer section 140. As a result, the consistency of the web 102 is increased from about twenty percent solids just prior to the press nip 130 to between about thirty percent solids and about fifty percent solids just after the press nip 130. The Yankee dryer section 140 comprises, for example, a steam filled drum 142 (“Yankee drum”) and hot air dryer hoods 144, 146 to further dry the web 102. The web 102 may be removed from the Yankee drum 142 by a doctor blade 152 where it is then wound on a reel (not shown) to form a parent roll 190.
A CWP papermaking machine, such as papermaking machine 100, typically has low drying costs, and can quickly produce the parent roll 190 at speeds from about three thousand feet per minute to in excess of five thousand feet per minute. Papermaking using CWP is a mature process that provides a papermaking machine having high runability and uptime. As a result of the compaction used to dewater the web 102 at the press nip 130, the resulting paper product typically has a low bulk with a corresponding high fiber cost. While this can result in rolled paper products, such as paper towels or toilet paper, having a high sheet count per roll, the paper products generally have a low absorbency and can feel rough to the touch.
As consumers often desire paper products that feel soft and have a high absorbance, other papermaking machines and methods have been developed. Through-air-drying (TAD) is one method that may result in paper products having these characteristics. FIG. 2 shows an example of a TAD papermaking machine 200. The forming section 230 of this papermaking machine 200 is shown with what is known in the art as a twin-wire forming section and it produces a sheet similar to that produced by the crescent former (forming section 110 of FIG. 1). As shown in FIG. 2, the furnish is initially supplied in the papermaking machine 200 through a headbox 202. The furnish is directed by the headbox 202 into a nip formed between a first forming fabric 204 and a second forming fabric 206, ahead of forming roll 208. The first forming fabric 204 and the second forming fabric 206 move in continuous loops and diverge after passing beyond forming roll 208. Vacuum elements such as vacuum boxes, or foil elements (not shown) can be employed in the divergent zone to both dewater the sheet and to ensure that the sheet stays adhered to second forming fabric 206. After separating from the first forming fabric 204, the second forming fabric 206 and web 102 pass through an additional dewatering zone 212 in which suction boxes 214 remove moisture from the web 102 and second forming fabric 206, thereby increasing the consistency of the web 102 from, for example, about ten percent solids to about twenty-eight percent solids. Hot air may also be used in dewatering zone 212 to improve dewatering. The web 102 is then transferred to a through-air drying (TAD) fabric 216 at transfer nip 218, where a shoe 220, for example, presses the TAD fabric 216 against the second forming fabric 206. In some TAD papermaking machines, the shoe 220 is a vacuum shoe that applies a vacuum to assist in the transfer of the web 102 to the TAD fabric 216. Additionally, so-called rush transfer maybe used to transfer the web 102 in transfer nip 218 as well as to structure the web 102. Rush transfer occurs when the second forming fabric 206 travels at a speed that is faster than the TAD fabric 216.
The fabric 216 carrying the paper web 102 next passes around through-air dryers 222, 224 where hot air is forced through the web to increase the consistency of the paper web 102, from about twenty-eight percent solids to about eighty percent solids. The web 102 is then transferred to the Yankee dryer section 140, where the web 102 is further dried. The sheet is then doctored off of the Yankee drum 142 by doctor blade 152 and is taken up by a reel (not shown) to form a parent roll (not shown). As a result of the minimal compaction during the drying process, the resulting paper product has a high bulk with corresponding low fiber cost. Unfortunately, this process is costly to operate because a lot of water is removed by expensive thermal drying. In addition, the papermaking fibers in a paper product made by TAD typically are not strongly bound, resulting in a paper product that can be weak.
Other methods have been developed to increase the bulk and softness of the paper product as compared to CWP, while still retaining strength in the paper web and having low drying costs as compared to TAD. These methods generally involve compactively dewatering the web and then belt creping the web so as to redistribute the web fibers in order to achieve desired properties. This method is referred to herein as belt creping and is described in, for example, U.S. Pat. No. 7,399,378, 7,442,278, 7,494,563, 7,662,257, and 7,789,995 (the disclosures of which are incorporated by reference herein in their entirety).
FIG. 3 shows an example of a papermaking machine 300 used for belt or fabric creping. Similar to the CWP papermaking machine 100, shown in FIG. 1, this papermaking machine 300 uses a crescent former, discussed above, as the forming section 110. After leaving the forming section 110, the felt run 118, which is supported on one end by roll 108, extends to a shoe press section 310. Here, the web 102 is transferred from the papermaking felt 116 to a backing roll 312 in a nip formed between the backing roll 312 and a shoe press roll 314. A shoe 316 is used to load the nip and to dewater the web 102 concurrently with the transfer.
The web 102 is then transferred onto a creping belt or fabric 322 in a creping nip 320 by the action of the creping nip 320. The creping nip 320 is defined between the backing roll 312 and the creping belt or fabric 322, with the creping belt or fabric 322 being pressed against the backing roll 312 by a creping roll 326. In the transfer at the creping nip 320, the cellulosic fibers of the web 102 are repositioned and oriented. The web 102 may tend to stick to the smoother surface of the backing roll 312 relative to the creping belt or fabric 322. Consequently, it may be desirable to apply release oils on the backing roll 312 to facilitate the transfer from the backing roll 312 to the creping belt 322. Also, the backing roll 312 may be a steam heated roll. After the web 102 is transferred onto the creping belt or fabric 322, a vacuum box 324 may be used to apply a vacuum to the web 102 in order to increase sheet caliper by pulling the web 102 into the topography of the creping belt or fabric 322.
It generally is desirable to perform a rush transfer of the web 102 from the backing roll 312 to the creping belt or fabric 322 in order to facilitate transfer of the web 102 to creping belt or fabric 322 and to further improve sheet bulk and softness. During a rush transfer, the creping belt or fabric 322 is traveling at a slower speed than is the web 102 on the backing roll 312. Among other things, rush transferring redistributes the paper web 102 on the creping belt or fabric 322 to impart structure to the paper web 102 in order to increase bulk and to enhance transfer to the creping belt or fabric 322.
After this creping operation, the web 102 is deposited on a Yankee drum 142 in the Yankee dryer section 140 in a low intensity press nip 328. As with the CWP papermaking machine 100 shown in FIG. 1, the web 102 is then dried in the Yankee dryer section 140 and then wound on a reel (not shown). While the creping belt 322 imparts desirable bulk and structure to the web 102, the creping belt 322 may be difficult to use. As the creping belt or fabric 322 moves through its travel, the belt bends and flexes, resulting in fatigue of the belt or fabric 322. Thus, the creping belt or fabric 322 is susceptible to fatigue failure. In addition, creping belts and fabrics 322 are custom designed elements with no other commercial analog. They are designed to impart a targeted structure to the paper web, and can be difficult to manufacture, since they are a low volume element and little prior commercial history exists. Additionally, the patterns and types of structures that can be imparted to the web 102 by a woven fabric 322 are limited because of constraints resulting from belt design and construction. Further, the speed of the papermaking machine 300 is slowed by the crepe ratio when the web 102 is rush transferred from the backing roll 312 to the creping belt or fabric 322. The slower exiting web speed leads to lower production speeds as compared to non-belt creped systems. Additionally, such creping belt runs require large amounts of floor space and thus increase the size and complexity of the papermaking machine 300. Furthermore, uniform, reliable sheet transfer to the creping belt or fabric 322 may be challenging to achieve. Accordingly, there is thus a desire to develop methods and apparatuses that are able to achieve the paper qualities comparable to those provided by fabric creping, but without the difficulties of the creping belt.